The primary purpose of this investigation is the development of methods of assessing habitual physical activity in epidemiologic studies and in smaller patient populations. Thirty males, age 25-55, will be used in this investigation. The energy expenditure during a seven-day period will be estimated by doubly labeled water as described by Schoeller and Webb. Subjects will ingest 12 mmol 18-0/kg estimated total body water and 10 mmol 2H/kg estimated total body water. From urine samples at the start and end of the 7 day period, mean daily CO2 production may be estimated from the isotope elimination rates. Energy expediture can then be estimated from CO2 production. During the week in which CO2 production will be estimated, the subject will wear three portable accelerometers from arising in the morning until retiring at night. The accelerometers will be mounted at right angles to each other so accelerations in three planes may be estimated. The principal aim will be to determine how accurately the portable accelerometers estimate energy expenditure. However, questionnaire-interview procedures, continuous heart rate recordings, and movement readings will also be employed to determine if any of these are useful for estimating energy expenditure.